The present invention relates to optical couplers, and, more particularly, but not exclusively to coupling between a source of light and a waveguide.
There is a growing demand for a large variety of display devices. Sophisticated display devices serve a growing number of applications with diverse requirements. There is a continuous pressure to improve covering all the aspects of the display.
In cathode ray tubes (CRT), plasma displays, light emitting diode (LED) displays, etc. each pixel radiates light independently. Display technologies such as liquid crystal display (LCD) do not produce light within the pixel and the pixel is only able to block light. Therefore a LCD display requires illumination behind the LCD device, known as backlight. Applications of backlight include devices as small as mobile phones and as large as wide-screen LCD TV sets. The main goals of the backlight design include: high brightness, large area coverage, uniform luminance throughout the illuminated area, controlled viewing angle, either wide or narrow, small thickness, low weight, low power consumption and obviously low cost. It is therefore clear that the backlight is key to the quality of the display.
A backlight device typically comprises a lamp and a light guiding fixture, named hereinbelow a waveguide. As seen in FIG. 2, the lamp 14 produces the light energy and the waveguide 15 carries the light from the lamp to the back of the LCD device 16 and distributes the light according to the requirements. The characteristics of the waveguide affect all the required characteristics of the backlight and the display: cost, size, brightness, uniformity, power consumption, weight, etc. A higher efficiency waveguide collects more light from the light source, delivers more light to the back of the display, distributes the light more evenly, is thinner, lighter and less expensive.
Waveguide technology exploits a physical phenomenon known as total internal reflection. This phenomenon occurs at an interface between two bulks of materials, one having a refraction index higher than the other. As can be seen in FIG. 3, when a ray of light 17 travels within the material 19 of the higher refractive index and impinges on the interface 18 at an angle larger than the critical angle (also known as the angle of total reflection) the entire light is reflected back 23 into the bulk of the larger refraction index. The abovementioned angles are measured from the perpendicular to the surface of the interface.
The condition according to which the light is reflected or refracted is determined by Snell's law, which is a mathematical relation between the ratio between the impinging angle and the refracting angle (in case in case of refraction) and the ratio between the refractive indices of the two interfacing materials. Broadly speaking, depending on the wavelength of the light, for a sufficiently large impinging angle no refraction can occur and the energy of the light is trapped within the substrate. In other words, the light is reflected from the internal surface as if from a mirror. Under these conditions, total internal reflection is said to take place. The critical angle αc is a function of the refraction indices n1 and n2 as follows:
      α    c    =            sin              -        1              ⁡          (                        n          1                          n          2                    )      
Wherein n2 is the refractive index of the material in which the light ray travels and n1 is the refractive index of the externally bounding material.
Therefore, for example and as seen in FIG. 4, a light pipe made of one material of relatively higher refractive index surrounded by another material of a lower refractive index, and illuminated at one side at a divergence angle smaller than required by the critical angle, carries all the light to its other end. The light follows the shape of the light pipe and emerges from the other end even if the pipe is bent, up to a certain curvature. A waveguide any assume various shapes, not necessarily like the example of the pipe. A backlight waveguide is usually flat, illuminated at its thin side and radiating from its large side, as can be seen in FIG. 2.
However, as is seen in FIG. 6, when a waveguide receives a light beam 30 that has a divergence angle 31 that is incompatible with the critical angle 22 the light that is outside the cone defined by the critical angle is refracted through the material 20 and outside the waveguide and is therefore lost.
Thus, a major aspect of the waveguide design is the interface between the lamp and the waveguide. The interface is designed to collect as much as possible of the visible energy produced by the light source. In most cases it is the waveguide that is designed to the lamp characteristics. Various lamps have various sizes, various spectral patterns, and various radiation patterns. Typical light sources are: florescent lamps, incandescent lamps, plasma lamps, light emitting diodes (LED), single fiber, fiber bundles, lasers, etc. An efficient integration of the lamp and the waveguide means that a greater portion of the visible light that the lamp produces is made available at the back of the LCD device.
To achieve efficient coupling the light emerging from the lamp must enter the waveguide within a specific divergence angle, meeting the waveguide's propagation angle. When the divergence angle complies with the propagation angle a larger portion of the light reaches the user, thus decreasing power consumption, increasing brightness, decreasing cost, and practically affecting all the parameters of the quality of the picture.
However, most of the light sources emit light over a divergence angle that is too large for all the light to be successfully coupled to the waveguide. Therefore there is a need for an additional optical system to bridge between the lamp and the waveguide. The additional optical system may typically operate by placing a larger input aperture over the light source, thus reducing the light loss due to divergence. Aside for the additional cost and size directly involved with the additional optical system, the aperture of the additional optical system is larger than the size of the light source, thus requiring a larger waveguide, increasing its weight and cost, but primarily its thickness. The coupling point between the light source and the waveguide is therefore an obstacle in making the backlight thinner and more efficient.
Another solution is the use of fiber optics bundles to provide the required coupling between the light source and the waveguide. The fibers are arranged around the light source to collect as much light within the fibers' propagation angle and are then curved and packed together to feed the light into the waveguide. Except for being expensive, this solution does not eliminate the problem because the light emerges at the end of the fibers at a divergence angle that is still too large for the waveguide.
The difference between a typical fiberoptic device and a typical backlight device should be noted. A typical fiberoptic device carries light from one end to the other. A good fiber optic device carries as much as possible light to the other end and loses as little as possible light through its sides. A typical backlight device receives light via one side, carries the light in a first direction and emits the light in a second direction. Typically the first and second directions are orthogonal. While most backlight devices are made of rigid waveguides, flexible waveguides are also known in the art, as taught by U.S. patent application WO 2004/053531A2, the contents of which are hereby incorporated by reference.
There is thus a widely recognized need for, and it would be highly advantageous to have, a waveguide coupling method and apparatus devoid of the above limitations.